@far_cryI would lie if I tell you a specific number but I'm sure it can handle more than a shockpump or a firdge compressor could provide and that's enough.

@LikimysCrotchus5Yea I know that I'm not the only one who build valves like that. I saw atleast two different people on youtube with simmilar valves (both french).So that I'm the minority....let's call it french valve

How does the piston work? Whether or not the gas input is pilot or chamber side, how does it get to pilot area from chamber or chamber from pilot area? basically, how does it flow past the piston if the two gaskets are sealing it...

If the second set of washers apparently doesn't seal 100% and a equalization hole can be put there, then what is the point of them? If im understanding this design fully, then couldnt it work just fine with just the hose-barb sealing gasket on the front of the piston? Or is there a purpose in trying to seal the pilot port?

Thanks for the how-to!It looks similar something I'm putting together in a 3/4" "T".Only I used some extra and different pieces, a female threaded 3/4"-1/2" reducer bushing, a 3/4" close nipple and a 1/2" nipple pipe threaded from inside "T" and inside the close nipple into back of coupling (rather than a hose barb).

It's longer to contain a Blow-forward bolt inside.

I'll probably refer to this for a similar piston.My least favorite part is the "T" ...it's cast...just like a hand grenade ...But I plan to eventually hydrotest it with a grease gun and vegetable oil, like Gippeto mentioned doing, and make sure what it can handle.

The brass sizes are confusing because the "1/8"" brass is ID .25, 1/4" is ID .375" the the 3/8" is ID close to .5" etc.

I'm a while from posting pics yet, but hopefully it will come together like my motorcycle, I curse and curse and before I even know it, it's finished.

@spencetron Of course you don't want a 100% sealing piston. The advantage of this piston design is, that it is adjustable. The 3/4" gaskets are smaller than the piston housing, so you put them between the two washers and squeeze them till they fit perfect. I should have written "100% fitting" instead of "sealing".

@boom_o_matic_2.0 and THUNDERLORD I've discovered that "hard rubber door stopper" make excellent piston bodies. You can squeeze them, they are light, take a lot of force (no bumper needed) and are available in different sizes.

I went to Lowes today with your parts list, wanting to make a slightly downsized version of your piston valve for my -in the making- blowdart gun. But as I was searching for the right hose barb I noticed that the hose barbs at my lowes were very different than the one you got. They didn't have any male threaded ones that weren't hexagonal shaped, so it wouldnt work with them. There were other ones that were female threaded and consisted of three parts. The hosebarb, a threaded bushing, and gasket. so I found one that would work if I sandwiched the hosebarb part between two 3/4" npt x mpt adapters. But it is a lot of metal, made centering the hosebarb pretty difficult, had a relatively small orifice and perhaps could have caused leaking at higher pressures.

So that is the frontal part of the piston valve that contains the sandwiched hosebarb part. Now after I screwed all that on tightly, I found an amazing little fitting that will solve all those problems under a heap of junk on my workbench. It was part of a "garden hose repair kit" apparently bought at walmart. It can be screwed on directly to the tee nice and neat and has a much larger orifice than the previous hosebarb setup..

here is that cool hosebarb piece and the front part of the piston. I opted to use a rubber stopper as the sealing face. I still need to build the rear end though.

Your how to guide here has really clarified a lot for me in terms of homemade piston valves. I was just going to get a QEV, but saw this guide and tried to tackle it. I still have more work to do on the piston, but im confident it will work well. Great how to, Thanks man.

Yea these hose barbs seem to be pretty rare but I have found my personal "fitting wonderland". It's an old fashioned hardware store/firm (150 years of family business) where you can find things that you get nowhere else.

@spencetron Did you know that you don't need a hose barb? The only reason that I use them is to attach multiple barrels. If you only use one barrel you can lead the barrel into the tee with a compression fitting.

I know I don't necessarily need to use hose barbs. But I don't know if I'm going to use .40" or .50" blowgun darts for my cannon. Ill need to buy samples of both darts and see what kind of metal pipe fits which dart best, I wouldn't trust the thin-walled blowgun barrels on a 400-500 psi cannon. So having the option of barrel swaps is very convenient in my design.

Gotta love those weird, small, traditional hardware stores. For one of my older cannons I couldn't find a sprinkler valve anywhere locally. But the old family run hardware store down the street somehow stocked orbit watermasters.